1. Field of the Invention
This present invention relates to an electrophotographic or electrostatic image forming apparatus wherein a developed image (toner image) is formed on an image bearing member and the toner image is transferred onto a transfer sheet to obtain an image. More particularly, this present invention is preferably embodied as a multi-color image forming apparatus having a transferring device for sequentially transferring a plurality of color toner images sequentially formed on the image bearing member onto the transfer sheet to superimpose the toner images on the transfer sheet.
2. Related Background Art
In conventional transferring devices used in the image forming apparatus of this kind, an electrostatic transfer process such as a bias roller transfer process or a corona transfer process has been generally utilized.
In the bias roller transfer process, the toner image is transferred onto the transfer sheet, by applying the transfer bias voltage having a polarity opposite to that of the charge of the toner image formed on the image bearing member to a transfer roller having a conductive layer acting as a transfer sheet bearing member. Alternatively, in place of the transfer roller, an endless belt having the conductive layer may be used.
On the other hand, in the corona transfer process, a dielectric film such as a polyester film is used as the transfer sheet bearing member, a transfer drum is formed by wrapping the film on a cylinder having a large undercut on its peripheral surface, and the toner image is transferred onto the transfer sheet by applying the corona discharge to the film from the inside of the transfer drum. Alternatively, in place of the transfer drum, an endless belt made of a dielectric film may be used.
In the conventional transferring device utilizing the above-mentioned bias roller transfer process, after the transferring operation has been finished, when the transfer sheet is separated from the transfer roller, there arises the discharge in the air (atmosphetic discharge), thus scattering the toner image on the transfer sheet, with the result that the image is distorted.
To cope with this problem, in the above-mentioned corona transfer process, for example, as disclosed in the Japanese Patent Laid-Open No. 61-117581, when the transfer sheet is separated from the transfer drum, the AC corona discharges are carried out from both transfer sheet side and the film side to remove the charges from the transfer sheet and the film, thereby reducing the atmospheric discharge during the separation of the transfer sheet to prevent the scattering of the toner image.
Incidentally, the bias roller transfer process has a structural advantage that the durability of the structure thereof is excellent and the structure is simple, in comparison with the corona transfer process; however, since the bias voltage is applied to the whole transfer roller, it is difficult to control the removal of the charge in the area where the transfer sheet is separated from the transfer roller, thus causing the scattering of the toner image easily.